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This article is part of a larger article titled "100+ Healthiest Foods On Planet Earth."  Read it here.

Onions Nutritional Information (per 100g)

Water: 89.1 g
Calories: 40 kcal
Protein: 1.1 g
Carbohydrate: 9.3 g
Dietary fiber: 1.7 g
Sugars: 4.2 g
Fat: 0.1 g
Vitamin C: 7.4 mg
Vitamin B3: 0.1 mg
Vitamin B6: 0.1 mg
Vitamin B9: 19 μg
Vitamin K: 0.4 μg
Calcium: 23 mg
Iron: 0.2 mg
Magnesium: 10 mg
Phosphorus: 29 mg
Potassium: 146 mg
Sodium: 4 mg
Zinc: 0.2 mg

Onions are one of most ubiquitous vegetables, and, luckily, may also be one of the healthiest. Easy to eat in bulk, and the basis of a number of delicious dishes, the onion makes a fantastic complement to a diet focused on a healthy lifestyle.

With a solid nutritional profile and extremely high levels of quercetin (more on that below), the onion is a fantastic introduction to the allium vegetables and a real benefit to your health.

Nutritionally, onions are simply never going to match something like, say, spinach. But considering how sweet they are when cooked and the large quantities it’s possible to eat them in comfortably, onions are by no means a bad choice for those looking to round out a healthy and nutritious diet.

The USDA database entry states that one large raw onion (150g), contains 18% of your daily value (DV) of vitamin C, 10% DV of dietary fibre, 10% DV of vitamin B6, and 6% DV of potassium, really quite significant for a very sweet vegetable that contains only 60 calories.

All of these nutrients are beneficial for avoiding deficiency and improving health. Both vitamin C and potassium have links with increased cardiovascular health, and B6 is important for haemoglobin production, essential for a functioning cardiovascular system. Moreover, vitamin C is essential for things like iron absorption (an essential mineral), and dietary fibre is an important part of a healthy digestive system. What all these isolated examples show is that eating onions is great for your health, provided your diet contains a number of other vegetable sources.

Onion has been linked with far more health benefits than bringing people up to nutritional par, however. This rather broad review notes the anti-bacterial, antioxidant and anti-inflammatory benefits of onions and garlic, which is certainly significant.

It is perhaps most interesting to note that that onion has a particularly high concentration of a flavonoid called quercetin.

Why is this important? Well, high concentrations of quercetin are uncommon outside things like tea and wine (which aren’t so good on the nutritional front). More significantly, quercetin has been linked to a reduction of the risk of cardiovascular disease (the top killer in the developed world).

Quercetin has been shown to lower blood pressure, in addition to reducing platelet aggregation, a major contributing factor to atherosclerosis, the condition in which plaques build up on the inside wall of the arteries and restrict blood flow.

Quercetin may also have some athletic performance benefits. It is important to note here that the best way to ensure high quercetin levels when cooking is to sauté the onions.

Finally, as an allium, onion has a strong association with a lower risk of cancer, specifically, prostate cancer, stomach cancer, and oesophageal cancer. With onions being associated with a lower risk of cancer and cardiovascular disease, your long term health may really benefit from eating them regularly.